Fat and Slim Nucleosides and Nucleotides as Potential Aticancer and Antiviral Agents: These novel ring-expanded (fat) and ring-contracted (slim) nucleoside analogues, designed and synthesized in this lab, are potential chain-terinators of nucleic acid synthesis when incorporated into a tumor or viral DNA/RNA during transcription (or reverse transcription in case of retroviruses including HIV that causes AIDS). Mode of chain termination, and hence the viral or tumor replication, is believed to be the base-mispairing accompanied by considerable deviation of the base-sugar bond from the natural array. In addition, because of their unique structural, spatial, and conformational characteristics and constraints, fat and slim nucleosides/-tides are excellent probes for nucleic acid metabolism, structure, and function. While we use EI/CI/FAB Mass Spectrometry for routine structural analyses of small molecules, the MALDI TOF mass spectrometer is necessary for the molecular weight determination and purity of oligonucleotides derived from or incorporated with fat or slim nucleotides.